Extrusion devices which comprise at least one screw mounted for rotation in a barrel which barrel includes a feed zone, a pressure augmentation zone and a discharge zone, are known. Such devices are used for extruding thermoplastic plastics materials and rubber. The material to be extruded is usually fed into the extruder in solid form and is converted into a fused mass usually known as a melt. However, it is generally true that the melt must not be subjected to pressures and temperatures which are above certain values which are specific to the particular material being extruded. It is also desirable to prevent the quality of the extruded product from being adversely affected. In consequence, in the discharge zone of the extrusion device, the melt is frequently subjected to a pressure, but this generally produces an unsatisfactory discharge output. A melt pump, which is disposed between the discharge zone of the extrusion device and the extrusion head, permits this pressure to be increased and the discharge output is improved by so doing.
The most common type of melt pump includes a gear pump and an example of such as arrangement is disclosed in German Offenlegungsschrift No. DE-OS 38 42 988. Such arrangement is widely used because it also provides a comparatively good melt conveyance effect.
However gear pump arrangements are disadvantageous in that they are comparatively expensive and also add to the overall length of the extrusion device.
It must also be considered disadvantageous that such gear pumps have to be provided with their own drive means, the output of which is controlled in dependence upon the rate of rotation of the conveyer screws. It must also be remembered that additional energy is introduced into the melt during the extrusion process by a melt pump. To prevent the melt from becoming overheated and, hence, from becoming damaged, the temperature of the melt must not be allowed to exceed a predetermined limit temperature. This necessitates the provision of an elaborate temperature measurement and regulation system in the region of the conveyer screws and the gear pump.